Memory devices for pattern matching

ABSTRACT

Memory devices might include control circuitry that, when checking for a match of a stored digit of data and a received digit of data, might be configured to cause the memory device to apply a first voltage level to a control gate of a first memory cell of a memory cell pair, apply a second voltage level different than the first voltage level to a control gate of a second memory cell of that memory cell pair, determine whether that memory cell pair is deemed to be activated or deactivated in response to applying the first and second voltage levels, and deem a match between the stored digit of data and a received digit of data in response, in part, to whether that memory cell pair is deemed to be deactivated.

RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No. 16/517,846, titled “METHODS AND APPARATUS FOR PATTERN MATCHING,” filed Jul. 22, 2019, which is a Continuation of U.S. patent application Ser. No. 16/019,650, titled “METHODS AND APPARATUS FOR PATTERN MATCHING,” filed Jun. 27, 2018, now U.S. Pat. No. 10,622,072, issued Apr. 14, 2020, which is a Continuation of U.S. patent application Ser. No. 15/841,490, titled “METHODS AND APPARATUS FOR PATTERN MATCHING,” filed Dec. 14, 2017, now U.S. Pat. No. 10,141,055, issued Nov. 27, 2018, which is a Divisional of U.S. patent application Ser. No. 14/991,007, titled “METHODS AND APPARATUS FOR PATTERN MATCHING,” filed Jan. 8, 2016, now U.S. Pat. No. 9,875,799, issued on Jan. 23, 2018, which are commonly assigned and incorporated herein by reference in their entirety and which claim priority to U.S. Provisional Application No. 62/102,168, filed on Jan. 12, 2015, which is incorporated herein in its entirety by reference.

FIELD

The present disclosure relates generally to memory and in particular, in one or more embodiments, the present disclosure relates to methods and apparatus for pattern matching.

BACKGROUND

Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.

Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming of a charge storage structure, such as floating gates or trapping layers or other physical phenomena, determine the data state of each cell. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, cellular telephones, and removable memory modules, and the uses for flash memory continue to expand.

Flash memory typically utilizes one of two basic architectures known as NOR flash and NAND flash. The designation is derived from the logic used to read the devices. In NOR flash architecture, a logical column of memory cells is coupled in parallel with each memory cell coupled to a data line, such as those typically referred to as digit (e.g., bit) lines. In NAND flash architecture, a column of memory cells is coupled in series with only the first memory cell of the column coupled to a bit line.

Content addressable memories (CAM) are memories that implement a lookup table function in a single clock cycle. They use dedicated comparison circuitry to perform the lookups. CAM application are often used in network routers for packet forwarding and the like. Each individual memory bit in a CAM requires its own comparison circuit in order to allow the CAM to detect a match between a bit of the key word (e.g., pattern) with a bit stored in the CAM. Typical CAM cells, then, use approximately nine to ten transistors for a static random access memory (SRAM)-based CAM, or four to five transistors for a dynamic random access memory (DRAM)-based CAM.

In a two cell NAND CAM, one bit of data is stored in a pair of cells, each cell being programmed to one of at least two data states (S0, S1). One bit of a pattern is coded as two read voltages on word lines (Vr0, Vr1) of the pair of cells. A combination of read voltages and cell states is used to determine a match/no-match condition for a bit, e.g., when the bit of the pattern is concordant with the data of a cell pair (e.g., cell pair does not conduct), a match may be indicated, and when the bit of the pattern is opposite of the data of the cell pair (e.g., cell pair conducts), a no-match condition may be indicated. Pass voltages are used to remove cell pairs in a string that are not to be part of a match operation, e.g., cause these cell pairs to conduct regardless of their data states. Typically, a determination of a match/no-match condition for a data pattern will examine a plurality of cell pairs connected in parallel to a bit line. If all cell pairs under examination match, then no conduction will be present on the bit line (e.g., a precharged data line), which will not discharge, indicating a match for the cell pairs under examination. If any cell pair under examination does not match, e.g., both cells of at least one cell pair conduct, the bit line (e.g., a precharged data line) is discharged, indicating a no-match condition.

FIG. 1 shows a basic two cell NAND CAM. Such a CAM is described in greater detail in U.S. patent application Ser. No. 13/449,082, filed Apr. 17, 2012, titled “METHODS AND APPARATUS FOR PATTERN MATCHING”. In a two cell NAND CAM 100, one bit of data can be stored as threshold voltages (representing data states) in two cells, 102 and 104. A logical 0 value for a bit of data programmed into two cells 102 and 104 can be represented by a first threshold voltage of 3 volts (e.g., within the data state S1) on the first cell (e.g., cell 102) and a second threshold voltage of 1 volt (e.g., within the data state S0) on the second cell (e.g., cell 104), and a logical 1 value for a bit of data programmed into two cells can be represented by a first threshold voltage of 1 volt (e.g., within the data state S0) on the first cell (e.g., cell 102) and a second threshold voltage of 3 volts (e.g., within the data state S1) on the second cell (e.g., cell 104). Although specific voltage levels are provided herein for ease of understanding, they are merely examples and may vary depending upon the particular memory technology utilized and other operation parameters.

A pattern to be matched in the memory is stored or received as well. Each bit of the pattern to be matched is represented by two voltages on word lines (e.g., Vr0 and Vr1). For example, a logical 0 value for a pattern bit may cause a voltage of 2 volts (e.g.,Vr0, a voltage sufficient to activate a cell having the S0 data state but not activate a cell having the S1 data state) to be applied to the gate of the first cell (e.g., cell 102) of the two cells, and may cause a voltage of 4 volts (e.g., Vr1, a voltage sufficient to activate a cell having the S1 data state) to be applied to the gate of the second cell (e.g., cell 104) of the two cells. A logical 1 value for a pattern bit may cause a voltage of 4 volts (e.g., a voltage sufficient to activate a cell having the S1 data state) to be applied to the gate of the first cell (e.g., cell 102) of the two cells, and may cause a voltage of 2 volts (e.g., a voltage sufficient to activate a cell having the S0 data state but not activate a cell having the S1 data state) to be applied to the gate of the second cell (e.g., cell 104) of the two cells. A comparison is made to a representation of data stored in the array, where each bit of stored data is also represented by two cells, each having its own programmed threshold voltage. A register may be used to store the pattern of bits, e.g., two bits of the register for each bit of the pattern.

With these threshold voltages and word line voltages, a no-match between the data stored in the cells 102 and 104 is determined when both cells conduct, and the bit line (e.g., precharged data line) for those cells discharges. When at least one cell does not conduct, a match condition is determined. FIG. 2 shows basic no-match 202 and match 204 conditions for a two cell NAND CAM. For the example of FIG. 2, cell 102 has the S0 data state and cell 104 has the S1 data state. As such, if voltage Vr0 is applied to word line WL0 and voltage Vr1 is applied to word line WL1, both cells 102 and 104 conduct as each voltage is sufficient to activate its respective cell 102 or 104. Conversely, if voltage Vr1 is applied to word line WL0 and voltage Vr0 is applied to word line WL1, cell 104 does not conduct as the voltage Vr0 is insufficient to activate cell 104. Although cell 102 would be activated in this situation, the cell pair 102/104 does not conduct as a result of their series connection.

A false match condition in NAND CAM cells can be a critical issue. A false match occurs when a cell that should be conducting moves to being a non-conducting cell. NAND memory has some inherent reliability issues. In a situation where pattern matching as in a NAND CAM memory is being performed, the reliability of NAND memory limits its application.

For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for increased reliability of NAND CAM memories.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram of a two cell NAND CAM portion of a memory array;

FIG. 2 is a diagram of matching in a two cell NAND CAM such as that shown in FIG. 1;

FIG. 3 is a schematic diagram of one embodiment of a portion of a memory array;

FIG. 4 is a diagram showing possible error sources in a CAM memory;

FIG. 5 is a diagram of a portion of a memory according to an embodiment of the disclosure;

FIG. 6 is a diagram of a portion of a memory according to another embodiment of the disclosure;

FIG. 7 is a flow chart diagram of a method according to another embodiment of the present disclosure;

FIG. 8 is a diagram of a portion of a memory according to another embodiment of the disclosure;

FIG. 9 is a flow chart diagram of a method according to another embodiment of the present disclosure; and

FIG. 10 is a block schematic of a memory in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

In the following detailed description of the embodiments, reference is made to the accompanying drawings that form a part hereof. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.

FIG. 3 illustrates a schematic diagram of one embodiment of a portion of a NAND architecture memory array 301 comprising series strings of non-volatile memory cells. The present embodiments of the memory array are not limited to the illustrated NAND architecture.

The memory array 301 comprises an array of non-volatile memory cells (e.g., floating gate) arranged in columns such as series strings 304, 305. Each of the cells is coupled in series (e.g., drain to source) in each series string 304, 305. An access line (e.g., word line) WL0-WL31 that spans across multiple series strings 304, 305 is coupled to the control gates of each memory cell in a row in order to bias the control gates of the memory cells in the row. Data lines, such as even/odd bit lines BLe, BLo, are coupled to the series strings and eventually coupled to sense circuitry (e.g., sense amplifier) (not shown) that detects the state of each cell by sensing current or voltage on a selected bit line. The bit lines BLe, BLo are also coupled to page buffers (not shown) that can be programmed by data from each selected word line. The sense circuitry and page buffers may be part of the same circuitry or the page buffers can be separate circuitry.

Each series string 304, 305 of memory cells is coupled to a source line 306 by a source select gate 316, 317 (e.g., transistor), and to an individual even or odd bit line BLe, BLo by a drain select gate 312, 313 (e.g., transistor). The source select gates 316, 317 are controlled by a source select gate control line SG(S) 318 coupled to their control gates. The drain select gates 312, 313 are controlled by a drain select gate control line SG(D) 314.

In a typical programming of the memory array, each memory cell is individually programmed as either a single level cell (SLC) or a multiple level cell (MLC). The cell's threshold voltage (Vt) can be used as an indication of the data stored in the cell. For example, in an SLC, a V_(t) of 2.5V might indicate a first data state while a V_(t) of −0.5V might indicate a second data state. An MLC uses multiple V_(t) ranges that each indicates a different data state. Multiple level cells can take advantage of the analog nature of a traditional charge storage cell by assigning a respective bit pattern to each of the data states.

When there is uncertainty in program/read/verify operations, a cell can be read as conducting when it is expected not to be conducting, or can be read as not conducting when it is expected to be conducting. Either instance may result in a match error. When a cell that is conducting becomes not conducting because of some error, that may be a more serious error. A cell that is not conducting becoming conducting is less an issue in pattern matching because such an error is only one conducting cell that should not be conducting, and a bit error would depend on all cells conducting when conduction is not expected, and patterns are typically quite large. The probability that one cell is reading incorrectly is referred to as the bit error rate (BER). Given a BER, the probability that a pair of cells, such as cells 102 and 104 described above, storing a bit of data matching a bit of the pattern becoming a pair of cells that do not match the bit of the pattern is equal to the BER. Basic error sources for a two cell NAND CAM are shown in FIG. 4.

One embodiment 500 for reducing the error rate for a NAND CAM is shown in FIG. 5. In this embodiment, a CAM cell is formed from four individual memory cells 502, 504, 506, and 508, comprising two pairs of two cells storing in one embodiment duplicate data. In one embodiment, cells 502 and 504 represent a bit of data like cells 102 and 104 of FIG. 1, and that bit of data is duplicated into cells 506 and 508 in series on the same string 510. Therefore, in this embodiment a single bit of data (e.g., corresponding to a particular bit position of a pattern of data) is represented by four cells, with each pair of cells (502/504 and 506/508) programmed to store the same bit of data. Note that although each cell pair is programmed to store the same bit of data (e.g., they have been subjected to programming operations seeking to adjust their threshold voltages to represent the same data values), errors may occur such that a particular cell pair may not contain its intended bit of data. Given a four cell CAM and a known BER, a probability that a match condition reads as a no-match condition is equal to the BER squared. Extending the number of cells used to represent a single bit exponentially decreases the probability of error. For example, extending to eight cells, the probability of a match condition reading as a no-match condition is BER⁴, and extending to N cells, the probability of a match condition reading as a no-match condition is BER^((N/2)).

Another embodiment 600 for reducing the probability of error due to BER is shown in FIG. 6. Embodiment 600 duplicates data along a bit line direction. In this embodiment, multiple cell pairs such as cells 102 and 104 are aligned in parallel along a same bit line. As shown, cells 602 and 604 represent a bit of data, and cells 606 and 608 represent a bit of data. Each pair of cells (602/604 and 606/608) in one embodiment are programmed to store the same bit of data (e.g., corresponding to a particular bit position of a pattern to be searched). Note that although each cell pair is programmed to store the same bit of data (e.g., they have been subjected to programming operations seeking to adjust their threshold voltages to represent the same data values), errors may occur such that a particular cell pair may not contain its intended bit of data. In this embodiment, the bits are stored in parallel along bit line 610. If either cell pair conducts, there is a no-match condition. If neither cell pair conducts, there is a match condition. Therefore, in order to change a no-match result to a match result, each cell pair must have an error in which a conducting cell becomes a non-conducting cell. If P is the probability that a conducting cell becomes non-conducting, then the probability of a no-match condition being read as a match condition is 2P*2P=4P². When reliability of a NAND CAM cell, which is related to the BER, becomes more important, a lower probability of error through the use of additional cells such as in the present embodiments may be used.

A method 700 of pattern matching in a NAND CAM is shown in flow chart form in FIG. 7. Method 700 comprises, in one embodiment, receiving a pattern to be searched in a memory in block 702, programming a plurality of cell pairs, each cell pair storing a bit of data and each cell pair of the plurality of cell pairs programmed to store the same bit of data in block 704, and checking for the pattern to be searched in the memory in block 706. The bit of data of the plurality of cell pairs corresponds to a particular bit position of a pattern to be searched. In one embodiment, where the cell pairs are aligned in parallel along a same bit line, determining that a match condition is met (e.g., for the particular bit position of the pattern to be searched) occurs when each of the plurality of cell pairs is non-conducting (e.g., indicating a match to the data value of the particular bit position), and determining that a no-match condition is met occurs when any of the plurality of cell pairs is conducting. In another embodiment, where the cell pairs are connected in series in a same string, determining that a match condition is met (e.g., for the particular bit position of the pattern to be searched) occurs when any of the plurality of cell pairs is non-conducting (e.g., indicating a match to the data value of the particular bit position), and determining that a no-match condition is met occurs when each of the plurality of cell pairs is conducting. Two cell pairs are used in one embodiment. However, should increased reliability be desired, additional cell pairs may be used. As shown in FIG. 5, in one embodiment the plurality of cell pairs are connected in series in a same string. As shown in FIG. 6, in another embodiment the plurality of cell pairs are aligned in parallel along a same bit line. Although depicted in linear fashion in FIG. 7, the method need not be performed in order. For example, programming the plurality of cell pairs may occur before and/or after receiving the pattern to be searched in the memory.

The pattern (e.g., key word) to be searched in one embodiment comprises bits, with each bit of the pattern programmed into (e.g., represented by) two bits of a register, and gate voltages are applied to gates of the respective cells of a cell pair of the memory responsive to the two programmed bits. Pattern checking on a cell pair further comprises in one embodiment applying programmed gate voltages of a selected bit of the pattern to be searched to the two respective cells of the stored data.

Another embodiment 800 for reducing match probability errors is shown in FIG. 8. The embodiment of FIG. 8 duplicates the same pattern 802 in a plurality of memory elements 801 on each of a plurality of bit lines 804, 806, and 808. Each memory element 801 represents one or more cell pairs representing a bit of data of the pattern 802. Thus, each memory element 801 of a set of memory elements of a particular pattern 802 corresponds to a particular bit position of a pattern to be searched, and each set of memory elements 801 corresponds to the same set of bit positions of the pattern to be searched. When using more than one cell pair for a memory element 801, the cell pairs of the memory element 801 can be connected in series, such as described with reference to FIG. 5, and/or in parallel, such as described with reference to FIG. 6. Note that although each memory element 801 is programmed to store the same bit of data (e.g., they have been subjected to programming operations seeking to adjust their threshold voltages to represent the same data values), errors may occur such that a particular memory element 801 may not contain its intended bit of data. For example, the memory element 801 coupled to data line 806 and shown in dashed line may not contain its intended data value. After the pattern 802 is duplicated, a majority check on matching for the duplicated patterns 802 may be performed. Checking is performed as normal for each pattern 802 on each bit line 804, 806, and 808. Whatever result, matching or no-matching, that is in the majority is assigned as the true result. For example, if the pattern 802 is duplicated on three bit lines 804, 806, and 808, then whatever indication, match or no-match, occurs on two or more, e.g., a majority, of the bit lines, is the accepted result. In the case of three bit lines, then a match on two bit lines indicates a match, and a match on three bit lines indicates a match. In the example of FIG. 8, even though bit line 806 indicates a no-match condition, a match condition would be indicated for the pattern 802 as bit lines 804 and 809 (e.g., a majority) indicate match conditions. In the general case, duplicating a pattern N times on N bit lines, and accepting a match when K or more patterns of the N patterns are matching, gives a final probability error that at most K-1 patterns are not matching of:

$\begin{matrix} {\sum\limits_{i = 0}^{K - 1}\;{\begin{pmatrix} n \\ i \end{pmatrix}{p^{i}\left( {1 - p} \right)}^{n - i}}} & (1) \\ {\begin{pmatrix} n \\ k \end{pmatrix} = \frac{n!}{{k!}{\left( {n - k} \right)!}}} & (2) \end{matrix}$

where p is the probability that one pattern has at least one matching error, and m is the key or pattern length, then

p=1−(1−BER)^(m)   (3)

A method 900 of pattern matching according to this embodiment is shown in flow chart form in FIG. 9. Method 900 comprises, in one embodiment, receiving a pattern to be searched in a memory in block 902, and checking for the pattern to be searched in the memory. Checking for the pattern further comprises in one embodiment programming data into a plurality of sets of memory elements, each set of memory elements coupled to a separate bit line of the memory, each set of memory elements programmed to contain the same data in block 904 (e.g., each corresponding to a same set of bit positions of the pattern to be searched), and checking for the pattern to be searched in the memory within each set of memory elements of the plurality of sets of memory elements in block 906. In one embodiment, a match condition is met when a majority of the bit lines coupled to the plurality of sets of memory elements indicates a match for the pattern to be searched. Programming data into a plurality of sets of memory elements in one embodiment comprises programming data into three sets of memory elements along three data lines. In another embodiment, a matching condition is met when the plurality of sets of memory elements numbers N, and when at least K sets of memory elements of the N sets of memory elements indicate a match. For one embodiment, K is greater than N/2. Although depicted in linear fashion in FIG. 9, the method need not be performed in order. For example, programming data into the plurality of memory elements may occur before and/or after receiving the pattern to be searched in the memory.

The methods described herein are orthogonal. Redundancy may be added to a NAND CAM in different ways, such as in the direction of a string, in the direction of a bit line, and/or in the direction of duplicating on bit lines. Embodiments of the present disclosure extend a number of cells in a string direction to decrease a probability that a match condition is read as a no-match condition, extend a number of cells in a bit line direction to decrease a probability that a no-match condition is read as a match condition, and/or repeat patterns on multiple bit lines for a majority matching check.

Methods described herein, such as shown and described with reference to FIGS. 3 and 5-9, may be performed by a memory, such as the example memory 1000 illustrated in FIG. 10. Referring generally to FIG. 10, the memory (e.g., a NAND flash memory device) 1000 typically comprises a memory array 1002 divided into columns (accessed by data lines 1004, such as those commonly referred to as bit lines) and rows (accessed by access lines 1006, such as those commonly referred to as word lines), a voltage generation system 1008, a controller 1010, input/output circuitry 1012 for command and data transfers, and a plurality of page buffers 1014. Each page buffer is coupled to one or more bit lines 1004 and has one or more data cache registers which are used to latch data sensed from the array during a read or verify operation, and to store data to be programmed into the array. The plurality of page buffers 1014 is further coupled to a bank 1016 of data detectors. The data cache registers of the plurality of page buffers 1014 may further store a pattern (e.g., data pattern) to be searched in, or programmed into, the memory 1000. The plurality of page buffers 1014 can perform basic logic functions, such as AND, OR, and NOT operations, on the data stored in its registers. Each page buffer of the plurality of page buffers 1014 is coupled to one data detector of the bank 1016 of data detectors. Each data detector provides circuitry to determine whether data in a corresponding page buffer indicates a match or no-match condition for a corresponding bit line 1004.

The controller 1010 may load a pattern to be searched into data cache registers of the plurality of page buffers 1014. The pattern may be received by the memory 1000 through the I/O circuitry 1012, such as might be received from an external device (e.g., a host device or memory controller) (not shown). The controller 1010 may then select voltages in response to the pattern to be searched, and apply those voltages, such as generated by voltage generation system 1008, to word lines 1006 to drive selected cell pairs coupled to bit lines 1004 to indicate their corresponding match/no-match condition depending upon the selected voltages applied to their control gates. Data indicative of the voltage responses of the bit lines 1004 indicating their corresponding match/no-match condition is stored in data cache registers of the plurality of page buffers 1014 for use by the bank 1016 of data detectors.

In searching for a pattern in the memory array 1002, the controller 1010 may begin searching at a starting address corresponding to a location in the memory array 1002, and may continue searching through incremented addresses until either a match is indicated or an ending address is reached. In programming a pattern of data into the memory array 1002, the controller 1010 may program a first data state into one memory cell of each cell pair of a plurality of cell pairs, and program a second data state into the other memory cell of each cell pair of the plurality of cell pairs for each bit position of the pattern. Which memory cell of a cell pair receives the first data state and which memory cell of the cell pair receives the second data state is determined by a data value of the corresponding bit position of the pattern to be programmed. Circuitry involved in carrying out methods described herein (e.g., controller 1010, bank 1016 of data detectors, plurality of page buffers 1014, etc.) may collectively be referred to as control circuitry. Although not necessary for an understanding of the embodiments described herein, a memory such as memory 1000 is described in more detail in U.S. patent application Ser. No. 13/449,082, filed Apr. 17, 2012, and titled “METHODS AND APPARATUS FOR PATTERN MATCHING.”

While one set of threshold voltages and gate voltages are described herein, it should be understood that as cell structures vary, or array and threshold voltages change, different sets of voltages may be used without departing from the scope of the disclosure.

Conclusion

Methods for increased reliability of matching/no-matching operations on a key-data pattern comparison, and memories using the methods have been described. In particular, some embodiments introduce redundancy in a data pattern to lower a probability of errors and improve manufacturability of devices.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof. 

What is claimed is:
 1. A memory device, comprising: an array of memory cells comprising a plurality of memory cell pairs, wherein each memory cell pair of the plurality of memory cell pairs is configured to be programmed to store a same digit of data corresponding to a particular digit position of key words to be searched in the memory device; and control circuitry for access of the array of memory cells; wherein the control circuitry, when checking for a match of a stored digit of data of the plurality of memory cell pairs and a data value of the particular digit position of a received key word, is configured to cause the memory device to: for each memory cell pair of the plurality of memory cell pairs: apply a first voltage level to a control gate of a first memory cell of that memory cell pair; apply a second voltage level, different than the first voltage level, to a control gate of a second memory cell of that memory cell pair; and determine whether that memory cell pair is deemed to be activated or deactivated in response to applying the first voltage level to the control gate of the first memory cell of that memory cell pair and applying the second voltage level to the control gate of the second memory cell of that memory cell pair; and deem a match condition to be met between the data value of the particular digit position of the received key word and the stored digit of data of the plurality of memory cell pairs when a number of the memory cell pairs of the plurality of memory cell pairs that are deemed to be deactivated is greater than or equal to a predetermined number of the memory cell pairs of the plurality of memory cell pairs that is less than a total number of memory cell pairs of the plurality of memory cell pairs.
 2. The memory device of claim 1, wherein the control circuitry is further configured to cause the memory device to deem a no-match condition between the data value of the particular digit position of the received key word and the stored digit of data of the plurality of memory cell pairs when the number of the memory cell pairs of the plurality of memory cell pairs that are deemed to be deactivated is less than the predetermined number.
 3. The memory device of claim 1, wherein the predetermined number is greater than the total number divided by two.
 4. The memory device of claim 1, wherein the control circuitry is further configured to cause the memory device to deem a match condition between the data value of the particular digit position of the received key word and the stored digit of data of the plurality of memory cell pairs when a majority of the memory cell pairs of the plurality of memory cell pairs are deemed to be deactivated.
 5. The memory device of claim 1, wherein the plurality of memory cell pairs comprises two memory cell pairs.
 6. The memory device of claim 5, wherein the plurality of memory cell pairs comprises three or more memory cell pairs.
 7. The memory device of claim 1, further comprising: a plurality of data lines; wherein each memory cell pair of the plurality of memory cell pairs is selectively connected to a respective data line of the plurality of data lines.
 8. A memory device, comprising: an array of memory cells comprising N strings of series-connected memory cells, wherein each string of series-connected memory cells of the N strings of series-connected memory cells comprises a respective set of memory cell pairs configured to be programmed to store a same pattern of data; a plurality of data lines, wherein each data line of the plurality of data lines is selectively connected to a first end of a respective string of series-connected memory cells of the N strings of series-connected memory cells; and a source, wherein the source is selectively connected to a second end of each string of series-connected memory cells of the N strings of series-connected memory cells; and control circuitry for access of the array of memory cells; wherein the control circuitry, when checking for a match between a pattern of data of a received key word and a stored pattern of data of each respective set of memory cell pairs of the N strings of series-connected memory cells, is configured to cause the memory device to: for each string of series-connected memory cells of the N strings of series-connected memory cells: for each memory cell pair of the respective set of memory cell pairs of that string of series-connected memory cells: biasing a control gate of a first memory cell of that memory cell pair to a first voltage level if a respective digit of the pattern of data of the received key word has a first data value, and biasing the control gate of the first memory cell of that memory cell pair to a second voltage level different than the first voltage level if the respective digit of the pattern of data of the received key word has a second data value different than the first data value; biasing a control gate of a second memory cell of that memory cell pair to the second voltage level if the respective digit of the pattern of data of the received key word has the first data value, and biasing the control gate of the second memory cell of that memory cell pair to the first voltage level if the respective digit of the pattern of data of the received key word has the second data value; determine whether that string of series-connected memory cells is deemed to be conducting or non-conducting between the source and its respective data line in response to biasing the control gates of the first memory cells and the second memory cells of each memory cell pair of the respective set of memory cell pairs of that string of series-connected memory cells; and deem a match condition to be met between the pattern of data of the received key word and the stored pattern of data of each respective set of memory cell pairs of the N strings of series-connected memory cells when a number of the strings of series-connected memory cells of the N strings of series-connected memory cells that are deemed to be non-conducting is greater than or equal to K strings of series-connected memory cells of the N strings of series-connected memory cells, where K is less than N and greater than zero.
 9. The memory device of claim 8, wherein the array of memory cells further comprises additional strings of series-connected memory cells, wherein each string of series-connected memory cells of the additional strings of series-connected memory cells has a first end selectively connected to a respective data line of the plurality of data lines, and has a second end selectively connected to the source.
 10. The memory device of claim 8, wherein K is greater than N/2.
 11. The memory device of claim 10, wherein N is an odd integer value.
 12. The memory device of claim 8, wherein the N strings of series-connected memory cells is a first N strings of series-connected memory cells, wherein the array of memory cells further comprises a second N strings of series-connected memory cells, wherein each string of series-connected memory cells of the second N strings of series-connected memory cells comprises a respective set of memory cell pairs configured to be programmed to store a same pattern of data different than the pattern of data that the first N strings of series-connected memory cells is configured to be programmed to store, and wherein each string of series-connected memory cells of the additional strings of series-connected memory cells has a first end selectively connected to a respective data line of the plurality of data lines and has a second end selectively connected to the source.
 13. The memory device of claim 12, wherein the control circuitry, when checking for a match between the pattern of data of the received key word and a stored pattern of data of each respective set of memory cell pairs of the second N strings of series-connected memory cells, is further configured to cause the memory device to: for each string of series-connected memory cells of the second N strings of series-connected memory cells: for each memory cell pair of the respective set of memory cell pairs of that string of series-connected memory cells: biasing a control gate of a first memory cell of that memory cell pair to the first voltage level if the respective digit of the pattern of data of the received key word has the first data value, and biasing the control gate of the first memory cell of that memory cell pair to the second voltage level if the respective digit of the pattern of data of the received key word has the second data value; biasing a control gate of a second memory cell of that memory cell pair to the second voltage level if the respective digit of the pattern of data of the received key word has the first data value, and biasing the control gate of the second memory cell of that memory cell pair to the first voltage level if the respective digit of the pattern of data of the received key word has the second data value; determine whether that string of series-connected memory cells is deemed to be conducting or non-conducting between the source and its respective data line in response to biasing the control gates of the first memory cells and the second memory cells of each memory cell pair of the respective set of memory cell pairs of that string of series-connected memory cells; and deem a match condition to be met between the pattern of data of the received key word and the stored pattern of data of each respective set of memory cell pairs of the second N strings of series-connected memory cells when a number of the strings of series-connected memory cells of the second N strings of series-connected memory cells that are deemed to be non-conducting is greater than or equal to K strings of series-connected memory cells of the second N strings of series-connected memory cells.
 14. A memory device, comprising: a plurality of data lines; a source; an array of memory cells comprising N sets of memory cell pairs each configured to be programmed to store a respective copy of a pattern of data, wherein each set of memory cell pairs of the N sets of memory cell pairs is selectively connected to the source and to a respective data line of the plurality of data lines, and wherein, for each set of memory cell pairs of the N sets of memory cell pairs, each memory cell pair of that set of memory cell pairs is configured to store a respective digit of its respective copy of the pattern of data; and control circuitry for access of the array of memory cells; wherein the control circuitry, in response to receiving a key word having a pattern of data to be searched in the array of memory cells, is configured to cause the memory device to: determine a set of control signals in response to a plurality of digits of the pattern of data of the key word; connect each set of memory cell pairs of the N sets of memory cell pairs to its respective data line and to the source; apply the set of control signals to each set of memory cell pairs of the N sets of memory cell pairs that were programmed to store respective copies of a particular pattern of data; for each set of memory cell pairs of the N sets of memory cell pairs, determine whether that set of memory cell pairs is conducting or non-conducting between its respective data line and the source; and deem a match condition to be met between the pattern of data of the key word and the particular pattern of data when a number of the sets of memory cell pairs of the N sets of memory cell pairs that are deemed to be non-conducting is greater than or equal to a predetermined K sets of memory cell pairs of the N sets of memory cell pairs, where K is less than N and greater than zero.
 15. The memory device of claim 14, wherein the set of control signals comprises two control signals for each memory cell pair of a set of memory cell pairs of the N sets of memory cell pairs, and wherein the control circuitry being configured to cause the memory device to apply the set of control signals to each set of memory cell pairs of the N sets of memory cell pairs comprises the control circuitry being configured to cause the memory device to apply a first control signal of the set of control signals to a control gate of a first memory cell of a particular memory cell pair of each set of memory cell pairs of the N sets of memory cell pairs and to apply a second control signal of the set of control signals to a control gate of a second memory cell of the particular memory cell pair.
 16. The memory device of claim 15, wherein each control signal of the set of control signals has a first voltage level or a second voltage level different than the first voltage level.
 17. The memory device of claim 16, wherein the first control signal has the first voltage level when a respective digit of the particular pattern of data for the particular memory cell pair of each set of memory cell pairs of the N sets of memory cell pairs has a first data value and has the second voltage level when the respective digit of the particular pattern of data for the particular memory cell pair of each set of memory cell pairs of the N sets of memory cell pairs has a second data value different than the first data value, and wherein the second control signal has the second voltage level when the respective digit of the particular pattern of data for the particular memory cell pair of each set of memory cell pairs of the N sets of memory cell pairs has the first data value and has the first voltage level when the respective digit of the particular pattern of data for the particular memory cell pair of each set of memory cell pairs of the N sets of memory cell pairs has the second data value.
 18. The memory device of claim 14, wherein N is greater than or equal to two.
 19. The memory device of claim 18, wherein N is greater than or equal to three.
 20. The memory device of claim 14, wherein K is greater than N/2. 